Method for obtaining corrosion rate of metals



March 19, 1957 G. A. MARSH 2,786,021

METHOD FOR OBTAINING CORROSION RATE OF METALS Filed April 21, 1955 HIGH VOLTAGE POWER SUPPLY rme MINUTES FIG. 2

'srloA NOLLVZIHV'IOd INVENTOR.

BY GLENN A. MARSH ATTORNEY 'tion, Serial Number 328,779;

a. rapid qualitative or,

=rosion on metallic. materials of construction;

METHOD non GBTAENING CORROSION RATE on METALS Glenn A. Marsh, Crystal Lake, Ill., assigncr to 'ihaiure Oil Company, Chicago, Ill., a'corporation or Ohio This invention relates to the use of-an ersetre'eiiemie'al 15 corrosion behavior and in ger'ieral these t'e' technique in the study of corrosion processes. It is 'm'ore specifically concerned with correlating the polari'za characteristics of metals with the corrosion rate'of said metals.

9JCS atent 2,786,021 Patented Mar, 19, 1957 5 tack and does not require the presence of moisture, the

second'type of corrosion requires the'pre s'enceofmoisture in the corrosive environment to produce a galvanic process. In this type of corrosion'there is consumed cheinical"efiergywiththe production of electricalenergy.

itthercfore should be apparent that-a knowledge of the electrochemical characteristics of corrosion cells would be useful in obtaining informatio as to" the cerr'osiuility o-ivarious substances. The prior art'doe's' disclose the use or electrochemical techniques as'a'meansflfor predicting 'iqueshave used; as aeriterion, the polarization characteristics of metallie materiais of construction. However none of fliese'techniques has been used successfully in'ra id qualitative and quantitative methods for investi ating The many complex and heterogeneous variables which c'orr'osibilit y and rate f Corrosion, l" r l 'm y may be involved in corrosion reactions make themvestiga-tion of corrosion problems very di fiicult. Although various attempts have been made to'simul c a'te corrosive environments and study the effect of'these environments under'conditions in which therat'es of corrosion reactions would be accelerated by increasing the intensity of the variables involved, as a general rule the results of these tests do not correlate veryreadily wi'th actual field data. As a result the time honoredtechbe obtained in a-r-natter of several minutes.- p I Polarization has been defined as the sienna-gs or the voltaic current from an electrolytic eelrriuerstuemstimulation. of dissociation products attire electrodes and 2s it. :is a measure of the irreversibility or an" electrolytic process; Polarization maybe observed by measuring the :ipotential. of anielec'trode-as referredto a referenceelectrode while current is assingthrdugu tire eieetrodewith the electrode immersed in anelectrolyte'. The'phoduots nique of exposing various metallic materials of constr'u'ced '3 t e el ro e S rf c b51186 tl' 6"1trod PO- tio'n-t'o actual conditions under which the metals are tobe used has provided information upon which the'hiost reliance may be placed. This technique has ohyi'ous disadvantages, such as the relatively long periods of time required to obtain sufficient information and the limitations in the technique due to the inability to-vary slight ly the environmental conditionsunder which the: metallic inaterials of construction are robe-employed. Themost desirable technique V which could be carried out either in the laboratory or in 40-.m the c rrodlng environment.

would be an accelerated process 'the solution to engineering"problems, occurring' due to C'GU'OSlOD, could be predicated. Ina copending applicafiled' December 30;. 1952, there isd'escrihed a polarization envelope technitiuelwhich investigations of electrochemical corrosiomcanbe: made It is therefore an object of this inv'e'ntioitto provide accelerated method for quantitatively determining: the

at best, a "semi-quantitative y 35' Re'rnoyal of thepolarization products 'byeithe'r' dissolving or diffusing away causes the electrode to be depolarized and the electrode potentialreturns to its normal electrode .potential. The term normal potential. i's inte'n'ded to designate the non-thermodynamic potential .of the metal It has been found that by correlating time with the depolarization: which occurs subsequentito the polarization produced by aniir'stantan'eous discharge of current, it is possible to produce: rapidly a relative potential-time curve which may b'e 'employed f in 'corrorsion studies.

Although the "production of cathodic and anodicdepolarization-time curves are knownelectrochemical tech- ..-niques, the separate cathodic and anodic' depolarization .efiects occurring after an instantaneous discharge of currentjhave never been studied in conjunction with each other to provide information which can be employed in corrosion studies prior to the polarization envelope technique described in my above ine'ntio'ne'd applications In ..-further applying these phenomena to the 'study of cor- :efiiect. of'electrochemical. corrosion by' means ofia rapid 5 1 have found that if the p a aflfldi' and technique which provides reproducible results. Itis a a further. object of'this: invention to correlate quantitatively the effect of electrochemical: polarization. with corrosion rate; Another obiect this invention isitoprovide-an electronic. apparatus which may be'enaployed in carryingjontinvestigations of." he corrosion r-a'te ofi metallic materials of construction exposed to-electrolytic corrosive environments.

tion data obtained byusing the apparatus showni in Figure l for investigating: the rate of: corrosion oisteel in. contact. with an. aerated brine'solutien. containingrio 70 used; .corrosiou inhibitor.

This type of. curveis hereinafter referred to as the depolarization-time curve.

cathodic depolarization-time curves are simnitaneou'sly produced after an initial polarizationis efiected'by means of an instantaneous discharge of current of sufiicient in- "tensity, and the respective variations "in electr'ode potential-occurring during the anodic and cathodic depolarization are algebraically added, the results may be graphically recorded by correlating the change iripo'tential with time to produce a composite curve showingithe fnet variationinpotential occurring during concurrent anodic and cathodic depolarization. From this curve an empirical ,quantitative relationship between corrosion rate and f-polariz ation can be derived- 7 p In-investigating corrosionrate-by meansofthis-tech- Ilnique, the apparatus shownrschematicall y -in Figure 1=-is v appajrtus consists essentiallyofia powersource oi high voltage direct current 19,. whichgis used-to-charge the capacitofll 'in a""current discharge mechanism 12,

cally in Figure able apparatus;

trodes 16 and 17 discharge mechanism shown in .whereby a direct current may the said electrodes. this objective by simply connecting across the electrodes through the circuit differences in electrode arization effech there would be traced 'repres'entation of this effect, 7 V

be" obtained with the oscilloscope, there are however cer-' a sensitive polarization detector 13, and a polarization meter 14. Cell is a receptacle which is adapted to contain an electrolytic corrosive solution to which the metallic material of construction being investigated is ex posed. Test electrodes 16 and 17 are disposed within cell 15 and immersed in the electrolyte to form a simple galvanic cell. The test electrodes, which are substantially identical in size, are prepared from the metal which is being studied. This cell is then connected in parallel between the current discharge mechanism 12 and the polarization detector 13. The apparatus, as shown schematil, represents the simplest form of a suit- "However the device may be made more flexible in its operation by providing a bank of capacitors and suitable-switches and relays in the current discharge mechanism which, as shown, consists simply of a double pole-double throw switch. Thus, in one preferred form, by means of a relay, the charged capacitor is permitted to discharge instantaneously across the two metallic elecof the cell 15. Although the capacitor Figure 1 of the accompanying drawing, as well as the aforementioned modifications of this mechanism, incorporates a capacitor for effecting a brief discharge of direct current'through the electrodes 16 and 17 in cell 15,

it is also within the scope of this invention to employ any other suitable means he briefly passed through For example, it is possible to achieve a source of high voltage direct current and, by means of a mechanical or electrical relay incorporated into the power source, apply the direct current produced from the power source 10 to the electrodes 16 and 17. The time interval during which the direct current from an external source is passed through the circuit to effect the measurable polarization of the test electrodes is the time that it takes for a capacitor or other equivalent source of high voltage direct current that can be rapidly discharged passage of current takes place in the course of about a few milliseconds. Accordingly, in the appended claims, the term instantaneous is employed to describe the brief passage of time during which the current from the external source of direct current is passing through the circuit. Other equivalent means will also be obvious to those skilled in the art. The polarization detector may be a vacuum tube voltmeter or any other suitable means containing the electrolytic cell. Thisv which will quickly recover from the high voltage to which it is initially subjected, and is sensitive to the small potential which are produced in the process. Similarly, any meter which is responsive to the output of the polarization detector may also be employed. In a preferred embodiment, a high speed 1e corder may be employed to graphically meter the changes in relative potential occurring during the short period of time required to depolarize the test electrodes. As an example of a suitable high speed recorder is a commercially available instrument which was used in this investigation. This apparatus was a strip chart potentiometer recorder having a range of 010 millivolts, a 1.5 second full scale balancing time, and a chart speed of about 2 inches per minute. Another means for automatically metering and recording the signal from the polarization detector would be polarization-depolarization with time in place of the high speed recorder described in the foregoing embodiment. Thus, by mounting a sensitized photographic plate or paper on the face of v polarization-depola suitable graphical to employ an oscilloscope" WhlCh was adapted to correlate the While a true picture may,

tion effects are both observed and environment with the discharging of the discharge is practically complete recorder to record the phenomena. In addition to the high speed strip recorder and the oscilloscope, a sensitive ammeter may also be employed. While an apparatus employing this expedient would permit the invention to be carried out in a less convenient manner than by means of the apparatus previously described, it is however possible to produce a graphical representation of the algebraic summation of cathodic and anodic depolarization. In employing the apparatus so modified, a sensitive ammeter is connected in the output circuit of a vacuum tube voltmeter. The vacuum tube voltmeter circuit is designed to prevent the power supply voltage from damaging the sensitive meter. The efiect of the polarization influences the flow of current in the output circuit of the polarization detector and, as a result, the polarization is made visibly manifest. Inasmuch as the potential difference between two electrodes is correlated with time, the current flow in the polarization output circuit produced by the polarization is visually observed and recorded as though occurring at zero time. The subsequent variation in current resulting from the depolarization is also visually observed and recorded at various time increments until the test electrode potentials return to their normal potential. Cathodic and anodic polarization-depolarizarecorded in this man ner, and the data thus obtained are employed to construct a graphical presentation of this effect.

In employing the apparatus of this invention to investigate the rate of corrosion of a metallic material of construction which is exposed to galvanic corrosion, test electrodes 16 and 17 are prepared from the metal which is being studied. These electrodes are substantially identical. It has been found that small diameter rods may be used, however small rectangular coupons are also suitable. After the electrodes have been cleaned and polished, they are placed in the cell 15, which has been filled with an electrolyte which simulates the corrosive which the metal would normally be in contact. The galvanic cell thus prepared is left to stand for about 10 minutes because this time is usually required for reproducible results. After 10 minutes, a composite depolarization curve is obtained by the following procedure: The capacitor 11 is charged by connecting it with the source of high voltage direct current 10; The charged capacitor is then connected between the electrodes 16 and 17 by suitably manipulating the current discharge mechanism. As a result, an instantaneous flow of current is produced across the electrodes by capacitor. The full power supply voltage does not show up on the recorder because the by the time'the recorder pen has moved a short distance. The variations in electrode potential which are thus produced at both the'an'ode and the cathode are simultaneously detected by also detected and the algebraic sum of the variations in potential recorded. The electrodes 16 and 17 are so arranged that the current from the current discharge mechanism flows from one to the other, thus making one electrode cathodic and the other anodic. When no current flows and under normal non-polarized conditions the electrode potential of both electrodes difference in potential between them is zero. However when current of suflicient intensity to effect polarization flows between the electrodes, they become polarized and a potential difference appears. The potential difference which is detected is the algebraic sum of the resultant cathodic and anodic polarization. By graphically recording the variations in potential occurring during the is the same, the

polarization-depolarization cycle, there is produced 'a I single composite curve which.

is a plot of the variation of the algebraic sum of cathodic and anodic polarization respect to time. Since depolarization}is occurring atther ur aceo h lectrodes. .-this-' mp site curve is. a depolarization-time -curve;

It ha's been found that a correlation exists between' the corrosion rate .of metals: exposed to, electrochemical corrosion and the configuration of thedepolarization-time curve. It is evident that the slopesofthe:depolarization: time curve should be the basis of thecorrelation. Possible criteria which may be-employed; in correlating corrosion rate are, (-'l);amount of polarization-remaining after a given time, (2) i difierence between 5 polarization voltagesat two fixedtimes, and, (3); time requiredgto depolarizetora certain fraction-ofthe maximum recorded polarization. These are illustrative of'possible criteria which might be used to correlate-theslope of the depolarization-time curvewithycorrosion. rate. Others however Willhe-evidentto thosewho are skilled in this particular and are also-considered within the-scope of this nv nt n.-

For the I purposes of illustrating the I instant; invention, rate of corrosion of. steel exposedto llllG SOlllfiODSiWfiS investigated and it was found thatinthisdnstance 4301",- relationexisted between the corrosiomra-te, and the time required to-depolarize to two-thirds "of the;-maximum recorded polarization. This. factor-"will hereinafter be e erred o :as s d is h W L I F g: 2.:iu t s manner. The fraction was chosen arbitrarily;- as. being most suit,- ableforthis investigation. However, ;in the. event that other corrosive environments are studied, the slopes of the depolarization-time curves shouldbevexaminedyto determine whether or not some other fraction might provide a better degree-ofcorrelation. Infieveloping a corre a i ro.m.- polar zation ata-,.. .seri sofrbrinesoluti n were amademp which contained idifierent'inhibitors in-prder thatithe'corrosion-ratecould be varied- Steel coupons of known weight were suspendeddn the solution in order to obtain actual weight loss data. These coupons, although immersed in the electrolyte, did not form a part of the polarization measurement system. Electrodes having substantially the same composition as the steel coupons were employed in the same solution to provide the polarization data. As a representative brine, a synthetic aqueous solution was prepared by dissolving 1.95 weight percent NaCl plus 0.49 weight percent MgCl2.6I-I2O in distilled water. This brine solution was air blown until saturated, the oxygen content of the brine then being 6.0 mg./l. Equal portions of this brine were placed in a plurality of beakers and 0.3 percent by weight of inhibitor was added to several of the beakers. Using these solutions as the electrolyte, the above described polarization experiments were conducted using a 100 microfarad condenser charged at 380 volts of direct current and graphical representations of the resulting polarization-depolarization phenomena, as illustrated by Figure 2, were obtained.

The foregoing operating data are only illustrative and non-limiting. In carrying out the instant invention the current density employed in the cell is an important operating variable. It should be sufiicient to cause measurable polarization but not high enough to cause a spark discharge across the electrodes 16 and 17 within the cell 15. I have found that the potential of the capacitor or other source of potential should be chosen so that an initial current density of from about 0.03 to 30 amp/cm. is produced, although in some instances current densities outside this range may be used. The time needed to obtain the composite anodic and cathodic polarizationdepolarization curve is ordinarily 3 to 5 minutes. This represents the maximum time required. Accordingly, shorter times may be used in some cases.

It is well known that the corrosion rate of steel in aerated brine may change with time. The polarization data should give instantaneous readings, that is, they should tell how fast the metal is corroding while the curve is being obtained. This means that in order to get an accurate correlation, the weight loss data would have 6 K to be obtained-overeat; very shortjtime. interval. proeedurewould introduce HIIOthEIiSODIWQOf error-memely, that of measuring verysmall weightdosses; Therefore, in-addition totheinitialcurve, similarrdatazswere obtained at severalperi'ods throughout thez'run. which lasted about 30 hours and an average forthezperiod of the run of about 30 hours was obtained. 'Actualsweight loss datawereobtained from; the/steel strips; which were immersed in'the electrolyte-during the run.

A plot of 5 versusthc actual corrosionrate ina'milligrams per square decimeterper-dayswasimadeusing a semilog scale :inor'de'r to OblIQlDQaZMHeBIiIflOt. The actual corrosion rate in irriilligrams persquare deciineter: per day will hereinafter; be referred to as ,---R.. From this plbt a linear relationship was computedby theiine thod-s of least squares and it: was found :,-that: the-following: relationship betweeno and R existed:for'steelexposedto aerated brine:

Table I contains a, tabular summary of the data "detained fromtlreseexperiments:

TABLE I.

Corrosion rate data and polarization datafor ste l-in aerated brme at room temperature T j R- No.- Inhibitor R, o =Expt.-

M-DD Sec. MDD 'lheon, MDD

1 Corrosion Inhibitor A con- 1.2 22.2 1.2 0

sisting of- 92% sodium oiy iiosp at a sodium ferrocyanide. 2...... Na .I... 13.3 1731 11.1 +212 3 NazB4On10H20 16:8 13.7 20.0 +3.2 4 Corrosion Inhibitor B" 21.8 13.7 20.0 +1:8

consisting .Of 4%;dodeeylg amine, tetradecylamine, 4% hexadecylamlne and 2% octadecenylamine. Corrosion Inhibitor C 25. 2 13. 7 20.0 +5. 2 NazOrOi 26. 5 10. 8 29.0 2. 4 Blank 32. 7 8.8 37. 0 +0. 7 Corrosion Inhibitor D 87.0 2.8 88 +4.0

By referring to Table I it is seen that the experimental r corrosion rates and the theoretical corrosion rates, as

determined by the instant invention, are substantially in agreement. This demonstrates the validity of this correlation and permits an accurate quantitative estimation of the polarization data from corrosion rate.

As it has been pointed out, different corrosive environments in contact with other metallic materials of construction subject to corrosion would produce other empirical relationships because of different corrosion mechanisms. For example, if in the foregoing illustrative description of the invention, oxygen-free brine were employed, the empirical relationship set forth above would probably not hold. In order to develop relationships which are applicable to different environments and difierent materials, it simply would be necessary to carry out the procedure as outlined above and correlate one aspect of the slope of the depolarizationtime curve with the corrosion rate to develop the necessary relationship.

It is thus seen that a quantitative correlation can be established between polarization data and corrosion rate. An extension of this technique would prove useful in making spot corrosion rate checks in pipelines or other installations which are in contact with a corrosive environment. By installing permanent fittings to which are attached two electrodes, the corrosion rate could be checked at any time by interconnecting the electrodes in an apparatus as described above. The use of this technique avoids the client of residual polarization caused by employing the same electrode for both anodic and cathodic data as well as eliminating the need for a reference electrode. This latter advantage makes possible the use of a simple, rugged installation which could be in- 07 sorted into pipeline and process systems to permit polarization data to be obtained under field conditions.

It will be understood that all changes or modifications of the examples of the invention herein chosen for the purposes of illustration which do not constitute departure from the spirit and scope of the invention are intended to be covered.

What is claimed is: r

1; Ina method for determining the corrosion rate of a metallic material of construction exposed to electro chemical corrosion, the steps which comprise passing an instantaneous discharge of direct current in an amount suflicient to efiect measurable polarization through a circuit containing a pair of substantiallyident-ical electrodes, prepared from said metallic material, immersed in an electrolytic solution, one electrode being the positive pole and the other: electrode being the negative pole, to polarize said electrodes and measuring and recording with respect to time the variations in potential difference between said electrodes during depolarization of said electrodesi i '2. A method for determining the corrosion rate of a metallic material of construction exposed to electrochemical corrosion, which comprises immersing in an electrolytic solution a pair of substantially identical electrodes prepared from said metallic material, one electrode being the positive pole and the other electrode being the negative pole, passing an instantaneous" discharge of direct current in an amount suflicient to effect measurable polarization of said electrodes through a circuit containing said electrodes to effect the polarization of each electrode, detecting electrically and metering with respect to time the variations in potential difference between said electrodes during the depolarization of said electrodes to produce a composite time-potential curve whereby the corrosion rate of said metallic material in contact with an electrolytic environment substantially similar to the 8 said electrolyticsolution can be correlated with aspects oftheslopeofsaid curve. I i

3. A method in accordance with claim 2 in which said net variationsin potential difference 'are graphically metered. y

'4. A method in accordance with claim 2 in which a current density of from about 0.03-30 amps. per square "centimeter is employed. V v

5. A method in accordance with claim 2 in which the maximum time required to meter said variations in potential difference is from about 3 to 5 minutes.

6. A method fordeterminingthe rate of corrosion of steel in contact with aerated brine which comprises immersing apair of substantially identical electrodes prepared from said metallic material, one electrode being the positive pole and the other electrode being the negative pole, passing an finstantaneous discharge of direct current in an amount sufficient to efi ect measurable polarization of said electrodes through a circuit containing said electrodes to effect the polarization of each electrode, detecting electrically and metering, with respect to time, flie variations in potential ditference between said electrodes during the depolarization of said electrodes to produce a composite time-potential curve, and determining the time required to depolarize from to percent of the maximum polarization whereby the corrosion rate of steel in contact with aerated brine may be computed from said determined time.

7. A method in accordance with claim 6 in which the said time is the time required to depolarize to about twothirds of the maximum polarization.

References Cited in the file of this patent 

1. IN A METHOD FOR DETERMINING THE CORROSION RATE OF A METALLIC MATERIAL OF CONSTURCTION EXPOSED TO ELECTROCHEMICAL CORROSION, THE STEPS WHICH COMPRISE PASSING AN "INSTANTANEOUS" DISCHARGE OF DIRECT CURRENT IN AN AMOUNT SUFFICIENT TO EFFECT MEASURABLE POLARIZATION THROUGH A CIRCUIT CONTAINING A PAIR OF SUBSTANTIALLY IDENTICAL ELECTRODES, PREPARED FROM SAID METALLIC MATERIAL, IMMERSED IN AN ELECTROLYTIC SOLUTION, ONE ELECTRODE BEING THE POSITIVE POLE AND THE OTHER ELECTORDE BEING THE NEGATIVE POLE, TO POLARIZE SAID ELECTRODES AND MEASURING AND RECORDING WITH RESPECT TO TIME THE VARIATIONS IN POTENTIAL DIFFERENCE BETWEEN SAID ELECTRODES DURING DEPOLARIZATION OF SAID ELECTRODES. 